Objective: To study whether levothyroxine (LT4) suppressive therapy exposes patients with differentiated thyroid cancer (TC) to an increased risk of osteoporosis.
Design and methods: Markers of bone formation (serum alkaline phosphatase (ALP), osteocalcin (OC), type I procollagen carboxyterminal (PICP) and aminoterminal (PINP) propeptide) and resorption (serum type I collagen carboxyterminal telopeptide (ICTP) and urine hydroxyproline (HOP)), as well as serum intact parathyroid hormone (PTH), 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D (1,25(OH)2-D) were measured in 29 patients (25 women, 4 men) with a median age of 45 years, and in 38 age- and sex-matched controls. In a subgroup of 14 patients the measurements were repeated after 5 weeks' interruption of LT4 therapy. Since the primary treatment of TC the patients had used TSH suppressive doses of LT4 (a mean daily dose of 215 microg) for 9 to 11 years. The bone mineral density (BMD) of patients and controls was measured by dual energy X-ray absorptiometry.
Results: When on T4 therapy, patients had significantly higher mean levels of ALP (+21%, P < 0.05), OC (+35%, P < 0.01), PICP (+10%, P < 0.05), PINP (+46%, P < 0.001), ICTP (+21%, P < 0.05), and HOP (+37%, P < 0.001) compared with controls. After stopping treatment, OC (-42%, P < 0.001), PINP (-7%, P < 0.05), and ICTP (-54%, P < 0.001) decreased, whereas PICP (+24%, P < 0.001) and 1,25(OH)2D (+29%, P < 0.01) increased. BMD of the lumbar spine and the upper femur was similar in patients and controls.
Conclusions: Patients with differentiated TC have high bone turnover when on LT4 suppressive therapy, After withdrawing treatment both bone formation and resorption decrease acutely. During development of hypothyroidism, serum PICP and PINP, which form from the same type I procollagen molecule and should change similarly, behaved differently. This may be due to different effects of hypothyroidism on their removal through separate receptors in the liver.